chore: drop devenv, take the toolchain from the host

.gitignore

@@ -23,11 +23,5 @@ test/fixtures/*
 .hex-home/
 .mix-home/
 
-# devenv runtime state (devenv.lock is checked in)
-.dev/
-.devenv/
-.direnv/
-devenv.yaml
-
 # Claude
 .claude/

AGENTS.md

@@ -22,8 +22,14 @@ change to a demux, mux, or codec path. CI runs it on every pull request
 and on every push to `main`.
 
 The first `task compile` builds the NIF from source and takes several
-minutes. devenv sets `EXMPEG_BUILD=1`, so a local build never pulls a
-precompiled artefact.
+minutes. The Taskfile sets `EXMPEG_BUILD=1`, so a local build never pulls
+a precompiled artefact.
+
+The toolchain comes from the host: the Elixir, OTP, and Rust versions in
+`.github/workflows/ci.yml`, plus FFmpeg 9 with its headers, `pkg-config`,
+and libclang for bindgen. On Arch that is `pacman -S ffmpeg clang`. A
+distro with an older FFmpeg builds 9 from source the way
+`.github/actions/setup/action.yml` does.
 
 ## Layout
 

README.md

@@ -134,7 +134,8 @@ To build the NIF from source, install Rust 1.98 or newer and set
 - Access to GitHub: the NIF builds on
   [our rsmpeg fork](https://github.com/rubas/rsmpeg) until an rsmpeg
   release on crates.io supports FFmpeg 9, so Cargo fetches it from there.
-- Rust 1.98+ for source builds.
+- Rust 1.98+ for source builds, and libclang for `bindgen`. Set
+  `LIBCLANG_PATH` when libclang is not in a default library path.
 - Elixir 1.17+ / OTP 26+ (the NIF targets Erlang NIF version 2.17).
 
 ## Runtime requirements (precompiled NIF consumers)

Taskfile.yml

@@ -4,6 +4,11 @@ version: "3"
 vars:
   CARGO_MANIFEST: native/exmpeg_native/Cargo.toml
 
+# Local builds compile the NIF from source; only Hex consumers fetch a
+# precompiled artefact.
+env:
+  EXMPEG_BUILD: "1"
+
 tasks:
   setup:
     desc: Install Elixir deps